Different control arrangements have been developed for regulating operation of an inverter coupled to an electric motor. By regulating the frequency and amplitude of the inverter output voltage, the motor operating characteristics (such as speed, torque and horsepower) can be correspondingly regulated. Certain operating environments pose difficult problems, especially in regulating system start-up when a large induction motor is driven by the inverter, and in correcting the bus voltage supplied to the inverter to compensate for changes in inverter operating frequency. Such problems have been encountered, for example, in a refrigeration system including a centrigual compressor with adjustable guide vanes, of the type shown generally in FIG. 1 of U.S. Pat. No. 4,151,725, entitled "Control System for Regulating Large Capacity Rotating Machinery", which issued May 1, 1979 to the assignee of the present invention. The teaching of that patent was principally directed to the capacity control system 50. There remained problems in conjunction with the logic control system as noted above, and it is to the solution of these problems that the present invention is particularly directed.
It is a principal object of the present invention to provide a logic control system for an inverter-driven motor of large capacity, effective to obtain precise start-up of the system and subsequent accurate regulation of the system operation.
Another important object of the invention is the provision of means for operating the inverter at a certain minimum frequency during system start-up, and thereafter bringing the inverter operating frequency smoothly up to its normal value.
Still another important object of the present invention is the provision of a logic control system suitable for interaction with a capacity control system of the type disclosed in U.S. Pat. No. 4,151,725, regulating not only the inverter-driven motor but also the adjustable inlet guide vanes of the compressor.